High speed keyer



Oct. 16, 1934.

5. G. ELLIS HIGH SPEED KEYER Filed se tf so. 1952 nugggggnu WITNESSES:

WA Wm INVENTOR Patented Oct. 16, 1934 UNITED STATE PATEN OFFIE HIGH SPEED KEYER Pennsylvania Application September 30, 1932, Serial No. 635,522

6 Claims.

This invention relates to facsimile transmission systems and particularly to systems especially adapted for the transmission of typewriting, printed matter, line drawings, or other objects presenting two contrasting degrees of brightness and not requiring the transmission of half-tones.

More specifically, my invention relates to that portion of a transmission system which is controlled by the scanning device and acts upon the apparatus supplying energy to the radiator to completely stop or cause full operation thereof.

Itis an object of my invention to produce a system sensitive to the'difference between the characters to be transmitted and their background, and insensitive to gradual changes of illumination or of the character of the background.

. It is a further object of my invention to provide a device for comparing the average'brightness of the object scanned with the brightness of the particular spot on the object viewed by the scanner at any one moment.

It is a further object of my invention to provide a bridge or balanced network with an energystorage device by means of which a potential corresponding to the average signal strength is compared with a potential corresponding to the momentary signal strength.

It is a further object of my invention to provide a keying system in which the difference between the average signal strength and the momentary signal strength serves as a criterion. Thus instead of a critical value of signal strength which is constant or at least a constant percentage of the maximum value, I have a discriminatory signal strength which changes with a change in the average value. The difierence between the average value and the discriminatory value is constant in my system.

In carrying out this invention, I produce the usualintermittent illumination of a photo-cell by light from the object of which a facsimile is to be produced by the usual chopper orequivalent arrangement. I amplify and rectify the output from the photocell, using a full-wave rectifier and impress the rectifier output upon a Wheatstone bridge. Across one arm of the bridge I place a condenser or other integrating device which has the eifect of averaging potential, whereby the potential difference between the ends of this arm will be the average of the impressed potential difference over a considerable period.

7 Another condenser is used across another arm of the bridge. The time constant of the combination of the latter condenser with its arm is so short that this condenser serves only to prevent interruption of the rectified current by the action of the chopper, and its averaging effect extends over only a few cycles of the chopper frequency. The bridge compares the long-period average with the momentary value and the result of this comparison is impressed upon the grid of a vacuum tube which may be itself the transmitter or may be connected through amplifying tubes to a transmitter.

Further details of the construction and other objects of my invention will be apparent from the following description and the accompanying drawing in which the single figure is a diagrammatic representation of the circuits and apparatus.

The notched wheel 1 is intended to indicate the chopper through which light from the typewritten sheet or other object is received. The means for illuminating the sheet and for scanning it are of any usual or suitable kind and have not been represented in the drawing.

The light from the chopper is received upon a photocell 2 and determines what current will be delivered by the battery 3 of the photocell circuit to the primary 4 of the transformer. As many amplifiers as desired may be inserted between the photocell and the transformer. The secondary 5 is connected in the usual way to two rectifying tubes 6, constituting a full-wave rectifier. On the drawing, these tubes have been represented as of the indirectly heated type, but any rectifying tubes of suitable design may be employed. The positive output lead '7 of the rectifier is connected at the point 8 to a Wheatstone bridge and the negative output lead 9 is connected to the opposite end of the diagonal of the bridge from the point 8.

The other diagonal of the bridge is connected by the leads l1 and 12 to the grid and cathode, respectively, of a vacuum tube 13. The lead 12 is illustrated as connected to the middle point of the secondary of a filament transformer but this is not a necessary limitation. Any usual or suitable connection to the cathode will serve.

The plate supply for the tube 13 is tapped off at 14 from a potentiometer 15. The potentiometer is preferably supplied from two sources of rectified and filtered current, the positive terminal of one source of rectified current and the negative terminal of the other source being connected in common to the center 16 of the potentiometer.

The plate connection of the tube 13 is through the shield grid of a four-electrode tube 17 and from there through a resistor 18 to the point 14 somewhat to the left of the center 16 of the potentiometer. The control grid and the cathode of the tube 17 are connected together and to a somewhat more negative point 19 upon the potentiometer, the cathode of the tube 13 being connected at 21 to. a still more negative point of the potentiometer.

The plate of the tube 17 is connected through a resistor 22 to a point 23 somewhat to the right of the center 16. The plate of the tube 17 is also connected with the grid of a tube 24, the plate of which is connected through a resistor to the positive end 26 of the potentiometer.

The connection to the transmitter is from the two ends of the resistor 25. The transmitter is of the usual type which requires for its most eflicient use that point 26 be grounded. The design of the transmitter also requires that. the resistor 25 be small compared with the resistors 18 and22 and for this reason the tube 24 must be a'power tube, frequently as large as watts. In the operation of the device, light from the chopper 1 acting upon the photocell 2 produces a regularly'fluctuating current in the primary 4 which results in a pulsating current delivered from the full-wave. rectifier'over. the leads 7 and 9. The chopper, in the form of the device most frequently used, has a frequency of 1200 interruptions per second. The pulsations over the leads '2' and 9 therefore have a frequency of 2400 pulsations per second. The arm of the bridge extending from the point 8 to the lead 1.1 is a resistor of some 10 megohms in parallel with a condenser 30 of some two microfarads. The other arm from the point 8 in the same embodiment of my invention comprises a resistor of 50,000 ohms in parallel with a condenser 31 of 0.01 microfarads. The other two arms of the bridge are designed to produce zero difference of potential across the diagonal between leads 11 and '12 when the'current is steady. This is accomplished by making the resistor between leads 9 and 11, 10 megohms and the resistor between leads 9 and 12, 50,000ohms. These dimensions are illustrative only, although they are the values use-d in one embodiment of the invention actually built. Any design of the bridge which gives a time constant for the arm between leads 7 and 11 of several seconds, and a time constant for the arm between leads 7 and 12 of a few cycles of the 2400 frequency will produce the results desired.

I have found it large enough for my purpose if the time constant of the condenser 31 is less than two cycles'but it maynot be as small as one cycle of the 2400 frequency.

' When the device is put into operation, the light reflected from the white background of a sheet of typewriting is steadily impressed 'upon the scanner. The chopper and photocell transform this into a pulsating current across the bridge which builds up a charge upon the condenser 30. The charge acquires a steady value in the course of a few seconds. There is also a steady value for the potential across the condenser 31 under these'circumstances. Under steady conditions, therefore, is no difference of potential across the diagonalof the bridge connecting loads 11 and 12. Thus there. is zero potential on the grid of the tube 13 and this tube is conductive. r

I As the scanner proceeds over the sheet it reaches the typewriting and upon arrival at the first portion of the first character, the magnitude of the. pulsations delivered over leads 7 and 9 suddenly diminishes, with the consequence that the potential across the condenser 31 falls in value, but because of the large time constant of the condenser 30, the potential there does not change substantially. The potential across condenser 31 and a discharge current flows counter clockwise around the bridge in opposition; to the potential across condenser 31. This current causes a drop along the lower arms of the bridge which makes the lead 11 negative with respect to the lead 12. That is, it places a negative potential upon the grid of the tube 13.

The negative potential just mentioned blocks the tube 13 and no more negative potential is needed than that required to block the tube. Consequently, even if the typewriting be faint, the change in signal strength when the scanner passes from the background to the first character, the tube will become blocked.

When the tube 13 ceases to be conductive, current no longer flows over the resistor 18 tothe tube 13. Consequently the. drop; over the re-' sistor 18 disappears and the potential of: the screen grid of the tube 17 is no longernegative but acquires a positive potential because the point 14 to which it is connected upon the potentiometer 15 is on the right of the point 19 to which the cathode of the tube 17 is connected... .The tube 1'7, therefore, becames conductive, although during the steady state it was non-conductive. i 105 Because the tube l'lis now conductive, a curerent flows from the point 23 over the resisto1ti22 and through the tube 17 to the point 19 of. the potentiometer. There is therefore, now a..drop over the resistor 22 which did not exist during a steady state.

In the steady state the grid of the tube 24 was positive, because the point 23 .is to. the right .of the point 16, but it is now negative and the tube 24 is thus blocked. The blocking of the tube 24 stops the current through the resistor. 25 which was present under steady conditions. This results in a change of the grid potential of the transmitter so that, instead of being. blocked, it now becomes active. 5

It thus appears that every'time that the scanner passes from white paper onto a character of the typewritten sheet, the transmitter changes from an inactive to an active condition. I

When the scanner has passed across the charactor and again reaches white paper, the charge upon the condenser 31 again builds up. There is, therefore, no longer a negative potentialupon the grid of the tube 13. This tube becomes conductive with the result that the tube 17 isblocked and the tube 24 becomes conductive, which results in the transmitter becoming again inactive. This process is repeated each time that the scan ner passes across a typewritten character.

A similar action will occur for. any sudden change in the'brightness of; that portion ofthe sheet toward which the scanner isdirected. Therefore, line drawings, printed matter and any other marking consisting of only two tone values can be transmitted by this system.

If the lamp illuminating the typewritten sheet changes its intensity, the result is a change in the value of the steady charge of the condenser 30', but no change results in the characteristic of the transmission because the tube 13: is .neither blocked nor unblocked; If the paper used be of a difierent quality, so that the backgroundalthough regarded as white is of a diiferent degree ofbrightness, the efiect is to make the. steady value .ofthe charge upon the condenser 30 .dif- 150 ferent but no loss of sensitiveness results because the keying depends not on the value of this charge but on the difference between it and the charge on the condenser 31.

When the end of the typewritten message is reached by the scanner, it proceeds over blank paper or over the uncovered surface or the cylinder upon which the typewritten sheet was mounted. No sudden changes in illumination, such as occurs when passing from background to character, is any longer present. Consequently, the tube 13 is in its steady state, that is, it is conductive with the result that the tube 17 is blocked, the tube 24 conductive and the transmitter inactive. This has the advantage that when the use of the transmitter for one message is finished, it no longer occupies the radio channel and other operators with other transmitters may use it without interference.

The sensitiveness of this transmission system is determined by the negative potential needed to block the tube 13. This tube may be so designed that a very small negative potential is sufficient for this purpose. The photocell and its associated amplifier or amplifiers can be designed to deliver through the rectifier a sufiicient voltage to work the bridge even when the characters are faint and upon a background of nearly the same brightness. At the same time great changes in the degree of illumination of the sheet and wide variations in the color or cleanliness of the sheets used can occur without interference with the effectiveness of the transmission system.

My invention is an improvement upon the invention disclosed in the application of L. R. Philpott Ser. No. 626,851, filed July 30, 1932, the most important diiference between the two inventions being that I have substituted a bridge for the series arrangement of resistors shown in said application with the result that I am able to produce the desired effect with a smaller number of tubes and with less costly apparatus. The apparatus using a bridge is also less bulky and weighs less, which is an advantage when it is used for communication with an aerial vehicle such as an airplane or a dirigible.

Many variations in detail will occur to those skilled in the art and the omission of specific mention of them is not to be construed as a limitation. The only limitations intended are those required by the prior art or indicated in the accompanying claims.

I claim as my invention:

1. In a facsimile transmission system, a viewing device, a keyer, and means therebetween for impressing on the keyer a potential corresponding to the difference between the average brightness of the object viewed and the brightness momentarily cooperating with the viewing device, said means comprising a network providing two conjugate conductors, one of said conductors extending to the keyer and the other to theviewing device.

2. In a facsimile transmission system, a viewing device, a keyer, and means therebetween for impressing on the keyer a potential corresponding to the difference between the average brightness of the object viewed and the brightness momentarily cooperating with the viewing device, said means comprising a condenser and resistor having a long discharge period and a condenser and resistor having a short discharge period.

3. In a facsimile transmission system, a viewing device, a keyer, and means therebetween for impressing on the keyer a potential corresponding to the difference between the average brightness of the object viewed and the brightness momentarily cooperating with the viewing device, said means comprising a network providing two conjugate conductors, one of said conductors extending to the keyer and the other to the viewing device, a resistor and condenser in parallel constituting one portion of said network.

4. In a facsimile transmission system, a lightsensitive device, means for delivering regularly intermittent light thereto, a rectifier associated with said light-sensitive device, a bridge energized by the output of said rectifier, an energy storage device associated with one arm of said bridge and a keyer-control tube the input of which is connected to the bridge in conjugate reiation to the output of the rectifier.

5. In a facsimile transmission system, a lightsensitive device, means for delivering regularly intermittent light thereto, a rectifier associated with said light-sensitive device, a bridge energized by the output of said rectifier, an energy storage device associated with one arm of said bridge and a keyer control tube the input of which is connected to the bridge in conjugate relation to the output of the rectifier, and a second energy storing device associated with that arm of the bridge which with the first-mentioned arm constitutes a connection across the input of the keyer control tube, one of said energy storing devices and its associated arm having a time constant not exceeding a few cycles of said regular intermissions and the other having a time constant exceeding many such cycles.

6. In a transmission system, a bridge, a vacuum tube the grid circuit of which is across one diagonal of said bridge, signal responsive devices for impressing rectified potentials across the other diagonal of said bridge, means in the arm of the bridge between the positive end or" the potentialreceiving diagonal and the grid for averaging the potential drop on said arm, whereby said potential can change only slowly and only in response to a change in the average signal strength, means in the other arm connected to said positive end of the potential-receiving diagonal for reducing variations in the potential drop on said arm in response to super-signal frequency while permitting changes in said potential drop in response to momentary changes of signal strength, whereby the tube will become blocked whenever the momentary signal strength is smaller than the average signal strength by more than a critical diiference but changes in the average signal strength will not alter the amount of said difference required to block the tube.

STUART G. ELLIS. 

